Earthquakes Lab 6

Concepts Diastrophism Earthquakes Types & causes of stress 5 types of folds 5 types of faults Earthquakes Focus, epicenter Types of energy waves Methods of measuring earthquakes Determining the magnitude of an earthquake Determining the epicenter of an earthquake Real-world example: New Madrid Fault

Diastrophism Definition: deformation of earth’s crust Deformation without movement Jointing: Fracture of rock without displacement Affects resistance of rock to erosion (weakens) Deformation with movement Folding: bending rock without breakage Faulting: fracture of rock with displacement (either vertical or horizontal movement)

Diastrophism Types & Cause of Stress 3 Types of Stress Compressional: Rocks move together (convergent motion) Tensional: Opposite movement (divergent motion) Shearing: Tearing (transform motion) Causes of stress Confining pressure Temperature Extreme heat  folds the rock without breakage Extreme cold  fractures the rock Strength/Composition of rock Time

Diastrophism 5 types of folds 1) Monocline: one-sided slope. Slight bend in otherwise parallel layers of rock. 2) Anticline: simple symmetrical upfold, resembles an arch. Due to compression. 3) Syncline: rock is warped downward – due to compression. 4) Overturned: upfold that has been pushed so vigorously from one side that it becomes over-steepened. 5) Overthrust: pressure was great enough to break the over-steepened area and cause a shear (a break).

Diastrophism 5 types of faults Normal: One block is displaced up, the other down. Due to tension. Reverse: A block is pushed up and over the other. Due to compression. Strike-slip: Adjacent blocks are displaced laterally. Movement is entirely horizontal. Due to shearing.

Diastrophism 5 types of faults (con’t) Graben: Subsidence of one middle block (it drops down). Due to tension. Horst: 2 reverse faults push a middle block up. Due to compression.

Earthquakes Atlanta New Madrid Fault

Earthquakes Sudden vibration within lithosphere from a quick release of energy Result of rock moving due to folding or faulting From point of origin (focus), energy is transmitted to surrounding rock by waves Focus: Origin of stress and energy release. Epicenter: Surface location of focus (directly above the origin).

Earthquakes Types of Energy Waves Body Waves Occur first. These are the initial waves emitted from the earthquake. These occur in a specific order. 1st wave: Primary “P” wave. 2nd wave: Secondary “S” wave. Surface Waves Occur after the body waves. These affect the surface of the earth (we typically feel these). Type 1: Love wave. Type 2: Rayleigh wave.

Earthquakes Types of Energy Waves: Body Waves Primary Wave (P wave) Expansion & contraction of rock as wave moves through it Fastest body wave Moves through solid rock and fluids (e.g., ocean/water) Secondary Wave (S wave) Wave moves through rock up and down and side-to-side Slower than P wave Can only move through solid rock

Earthquakes Types of Energy Waves: Surface Waves Love Wave Rolling/swaying effect on surface Moves the ground from side-to-side Fastest surface wave Rayleigh Wave Rolls along ground like an ocean wave Type most often felt during quakes

Earthquakes Methods of measuring earthquakes Modified Mercalli Intensity Scale Measures “intensity” of earthquake (e.g., the amount of shaking felt and the damage done). Very subjective: depends on the viewer’s description of the earthquake event! Based on observations. Richter Scale Measures the “magnitude” of earthquake (the energy waves released). Based on readings from a seismograph, and examining the actual energy waves.

Modified Mercalli Intensity Scale Richter Scale Modified Mercalli Intensity Scale Logarithmic Scale: Each increase in magnitude is 10x more energy released 5.0 is 10x greater than 4.0 5.0 is 100x greater than 3.0 5.0 is 1000x greater than 2.0 I Not felt II Felt only by persons at rest III/IV Felt by persons indoors only V/VI Felt by all: some damage to plaster/chimneys VII People run outdoors, damage to poorly built structures VIII Well-built structures slightly damaged, poorly-built structures suffer major damage IX Buildings shifted off foundation X Some well-built structures destroyed XI Few masonry structures remain standing, bridges destroyed XII Damage is total: waves seen on ground, objects thrown into air

Earthquakes Determining the magnitude of an earthquake A seismologist reviews data taken by a seismograph. Two important pieces of data to record: Lag Time: difference in time between the P wave and the S wave (when each is picked up by the seismograph). Designated as “S – P” Given in seconds. Amplitude: the size of the largest S wave (the height of the wave). Given in millimeters.

Result from a seismograph machine: Seismogram Earthquakes Determining the magnitude of an earthquake Result from a seismograph machine: Seismogram

Take the information from a seismogram (lag time and amplitude) Earthquakes Determining the magnitude of an earthquake Take the information from a seismogram (lag time and amplitude) and apply it on a nomograph.

Earthquakes Determining the location of an earthquake: Triangulation Receive data from at least three seismographs… Station 1: Eq occurred 10km from station Station 2: Eq occurred 5 km from station Station 3: Eq occurred 8 km from station Plot them all together, and your intersecting point is the epicenter!

Earthquakes Example: New Madrid Fault Earthquakes & aftershocks of 1811 and 1812 Tremors caused the Mississippi River to flow backwards Caused Reelfoot Lake to be formed Felt far away - steeple bells rang in Boston Sparse population in that region accounted for a lack of serious damage Richter scale had not been established, but would have registered 8.5 to 9.0 A projected 60% chance for a(nother) damaging earthquake by 2020 and 90% by 2040